MySQL——分组函数、distinct、分组查询、链接查询、子查询

时间 2019-12-06 标签 mysql 分组函数 distinct 查询链接

博客目录

一、分组函数

二、distinct

三、分组查询(group by、having)

四、一个完整的DQL语句的总结

五、链接查询(跨表查询)（关联多个表查询）

六、子查询

1、分组函数

一、会自动忽略空值

二、分组函数不能直接使用在where关键字后面

取平均值：avgmysql

mysql> select avg(sal) from emp;
+-------------+
| avg(sal)    |
+-------------+
| 2073.214286 |
+-------------+
1 row in set (0.60 sec)

最大值：maxweb
最小值：min

取记录数：count sql

mysql> select count(sal) from emp;
+------------+
| count(sal) |
+------------+
|         14 |
+------------+
1 row in set (0.04 sec)

求和：sum

2、distinct(去除重复记录)(显示)

使用distinct须要注意

一、该关键字前面不能出现字段

二、该关键字只能出如今全部字段的最前面

mysql> select distinct job from emp;svg

+-----------+
| job       |
+-----------+
| CLERK     |
| SALESMAN  |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
+-----------+
5 rows in set (0.09 sec)

例

统计公司一共有几个工做岗位函数

mysql> select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
|                   5 |
+---------------------+
1 row in set (0.00 sec)

3、分组查询(group by、having)

一、group by

经过哪一个或者哪些字段进行分组code

重点

若一条DQL语句中有group by子句，那么select关键字后面只能跟参与分组的字段和分组函数(好比ename这些是不能够的，不是分组字段)router

例

一、找出每一个工做岗位的最高薪水xml

分析：先按照工做岗位分组，再使用max求每组最高记录排序

mysql> select job,max(sal) from emp group by job;
    +-----------+----------+
    | job       | max(sal) |
    +-----------+----------+
    | ANALYST   |  3000.00 |
    | CLERK     |  1300.00 |
    | MANAGER   |  2975.00 |
    | PRESIDENT |  5000.00 |
    | SALESMAN  |  1600.00 |
    +-----------+----------+
    5 rows in set (0.00 sec)

二、计算每一个部门的平均薪水ip

mysql> select
    -> deptno,avg(sal) as avgsal
    -> from
    -> emp
    -> group by
    -> deptno;
+--------+-------------+
| deptno | avgsal      |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)

三、计算不一样工做岗位的最高薪水

mysql> select
    -> deptno,job,avg(sal) as avgsal
    -> from
    -> emp
    -> group by
    -> deptno,job;
+--------+-----------+-------------+
| deptno | job       | avgsal      |
+--------+-----------+-------------+
|     10 | CLERK     | 1300.000000 |
|     10 | MANAGER   | 2450.000000 |
|     10 | PRESIDENT | 5000.000000 |
|     20 | ANALYST   | 3000.000000 |
|     20 | CLERK     |  950.000000 |
|     20 | MANAGER   | 2975.000000 |
|     30 | CLERK     |  950.000000 |
|     30 | MANAGER   | 2850.000000 |
|     30 | SALESMAN  | 1400.000000 |
+--------+-----------+-------------+
9 rows in set (0.04 sec)

二、having

having和where功能都是为了完成数据的过滤
where和having后面都是添加条件
where和group by以前完成过滤
having在group by以后完成过滤

例

找出每一个工做岗位的最高薪水，除manager以外

mysql> select
    -> job,avg(sal)
    -> from
    -> emp
    -> group by
    -> job
    -> having
    -> avg(sal)>1500;
+-----------+-------------+
| job       | avg(sal)    |
+-----------+-------------+
| ANALYST   | 3000.000000 |
| MANAGER   | 2758.333333 |
| PRESIDENT | 5000.000000 |
+-----------+-------------+
3 rows in set (0.09 sec)

注意上面为何不能用where，由于where关键字后面不能用分组函数，分组函数必须在分组完成后执行，而分组要group by。而group by在where后执行

原则

通常状况尽可能在where中过滤，实在没法过滤的数据，一般是须要先分组以后再过滤，此时能够选择having，这牵扯到效率问题

4、一个完整的DQL语句的总结

select   
    ...
from    从某张表中检索数据
    ...
where   通过某条件进行过滤
    ...
group by    分组
    ...
having      分组以后不满意再过滤
    ...
order by    排序输出
    ...

以上关键字顺序不能改变，严格遵照

如下是执行顺序

一、from
二、where
三、group by
四、having
五、select
六、order by

5、链接查询(跨表查询)(关联多个表查询)

一、链接查询分类(年代)

SQL92
SQL99

二、链接查询根据链接方式分类

内链接(两张表在进行表链接的时候，链接记录百分百匹配记录可查，内链接只能是两表匹配的)
- 等值链接
- 非等值链接
- 自链接
外连接(两表进行外链接的时候，除了能够百分百记录查询，另外须要将其中任意一张表数据无条件的所有展现出来，就要使用外链接)
- 左外链接(左链接)：将左边数据所有显示
- 右外链接(右链接)：将右边数据所有显示
全链接

外链接的查询结果都是一直大于等于内链接的查询结果

三、当多张表进行链接查询，若没有任何条件进行限制，会发生什么现象

会发生总记录是两张表记录乘积(此现象是笛卡尔积现象，为了不该现象发生，必须在表链接时候加上限制)

例

一、内链接（等值链接）—————查询每一个员工所在的部门名称，要求最终显示员工名和对应的部门名(两张表：员工表(部门号)、部门表(部门号))

tip：在链接查询的时候虽然使用了限制条件，可是匹配的次数没有减小，仍是与无限制同样，只是此次的结果都是有效记录

SQL92语法：（属于内链接中的等值链接）（缺点：表链接与过滤同时了，结构不清晰）

mysql> select
    -> e.ename,d.dname
    -> from
    -> emp e,dept d
    -> where
    -> e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.15 sec)

SQL99语法(inner能够省略)（好处：表链接与过滤分离，在最后加where便可过滤，结构清晰）

mysql> select
    -> e.ename,d.dname
    -> from
    -> emp e
    -> inner join
    -> dept d
    -> on
    -> e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

二、内链接（非等值链接，SQL99）————找出每个员工对应的工资等级，要求显示员工名，工资，工资等级（两张表：员工表，工资等级表(salgrade)）

mysql> select
    -> e.ename,e.sal,s.grade
    -> from
    -> emp e
    -> join
    -> salgrade s
    -> on
    -> e.sal
    -> between
    -> s.losal
    -> and
    -> s.hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.03 sec)

三、内链接(自链接,SQL99)——找出每一个员工的上级领导，要求显示员工名以及对应的领导名(一张表当作两张表)

如下是员工表，表中也有员工序号和对应上级的序号，便可看出两张表

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO
+-------+--------+-----------+------+------------+---------+---------+--------
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10
+-------+--------+-----------+------+------------+---------+---------+--------
14 rows in set (0.00 sec)



mysql> select
    -> a.ename as empname,b.ename as leadername
    -> from
    -> emp a
    -> join
    -> emp b
    -> on
    -> a.mgr=b.empno;
+---------+------------+
| empname | leadername |
+---------+------------+
| SMITH   | FORD       |
| ALLEN   | BLAKE      |
| WARD    | BLAKE      |
| JONES   | KING       |
| MARTIN  | BLAKE      |
| BLAKE   | KING       |
| CLARK   | KING       |
| SCOTT   | JONES      |
| TURNER  | BLAKE      |
| ADAMS   | SCOTT      |
| JAMES   | BLAKE      |
| FORD    | JONES      |
| MILLER  | CLARK      |
+---------+------------+
13 rows in set (0.00 sec)

SQL92语法就是将join改为’,’，on改为where

四、外链接(右链接)——找出每一个员工对应的部门名称，要求部门名称所有显示（部分多出来的记录将用null与其匹配）(左链接的话将dept和emp换个位置就好)

mysql> select
    -> e.ename,d.dname
    -> from
    -> emp e
    -> right outer join
    -> dept d
    -> on
    -> e.deptno=d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.05 sec)

五、找出每一个员工对应的领导名，要求显示全部的员工

mysql> select a.ename empname,b.ename leadername from emp a join emp b on a.mgr=
b.empno;
+---------+------------+
| empname | leadername |
+---------+------------+
| SMITH   | FORD       |
| ALLEN   | BLAKE      |
| WARD    | BLAKE      |
| JONES   | KING       |
| MARTIN  | BLAKE      |
| BLAKE   | KING       |
| CLARK   | KING       |
| SCOTT   | JONES      |
| TURNER  | BLAKE      |
| ADAMS   | SCOTT      |
| JAMES   | BLAKE      |
| FORD    | JONES      |
| MILLER  | CLARK      |
+---------+------------+
13 rows in set (0.00 sec)

（多张表）六、找出每一个员工对应的部门名称，以及该员工对应的部门名称，工资等级。要求显示员工名、部门名、工资等级

多张表进行表链接的语法格式(a,b,c表)
原理：a与b链接后，再a与c链接

select
    ...
from 
    a
join
    b
on
    条件
join
    c
on
    条件；

mysql> select e.ename,d.dname,s.grade from emp e join dept d on e.deptno=d.deptn
o join salgrade s on e.sal between s.losal and s.hisal;
+--------+------------+-------+
| ename  | dname      | grade |
+--------+------------+-------+
| SMITH  | RESEARCH   |     1 |
| ALLEN  | SALES      |     3 |
| WARD   | SALES      |     2 |
| JONES  | RESEARCH   |     4 |
| MARTIN | SALES      |     2 |
| BLAKE  | SALES      |     4 |
| CLARK  | ACCOUNTING |     4 |
| SCOTT  | RESEARCH   |     4 |
| KING   | ACCOUNTING |     5 |
| TURNER | SALES      |     3 |
| ADAMS  | RESEARCH   |     1 |
| JAMES  | SALES      |     1 |
| FORD   | RESEARCH   |     4 |
| MILLER | ACCOUNTING |     2 |
+--------+------------+-------+
14 rows in set (0.05 sec)

TIP

为何inner、outer能够省略

由于区份内链接和外链接依靠的不是这些关键字，而是看SQL语句中是否存在left/right，若存在，标识必定是个外链接，其余都是内链接

inner、outer加上有什么好处

加强可读性

6、子查询

子查询就是嵌套的select语句(select语句嵌套select语句)

一、子查询出现的位置

select…(select)
from…(select)
where…(select)

例

where后使用子查询

找出薪水比公司平均薪水高的员工，要求显示员工名和薪水

第一步找出公司的平均水平
```
select avg(sal) from emp;
```

第二步找出薪水大于平均薪水的员工信息

mysql> select ename,sal from emp where sal>(select avg(sal) from emp);
+-------+---------+
| ename | sal     |
+-------+---------+
| JONES | 2975.00 |
| BLAKE | 2850.00 |
| CLARK | 2450.00 |
| SCOTT | 3000.00 |
| KING  | 5000.00 |
| FORD  | 3000.00 |
+-------+---------+
6 rows in set (0.00 sec)

from后面使用子查询（可将查询结果当作临时表）

找出每一个部门的平均薪水，并找出平均薪水的薪水等级

第一步找出每一个部门的平均薪水

mysql> select avg(sal) as avgsal from emp group by deptno;

第二步将上面的查询结果当作临时表t，t表和salgrade s表进行表链接，条件：t.avgsal between s.losal and s.hisal

mysql> select
    -> t.deptno,t.avgsal,s.grade
    -> from
    -> (select deptno,avg(sal) as avgsal from emp group by deptno) t
    -> join
    -> salgrade s
    -> on
    -> t.avgsal
    -> between
    -> s.losal
    -> and
    -> s.hisal;
+--------+-------------+-------+
| deptno | avgsal      | grade |
+--------+-------------+-------+
|     30 | 1566.666667 |     3 |
|     10 | 2916.666667 |     4 |
|     20 | 2175.000000 |     4 |
+--------+-------------+-------+
3 rows in set (0.12 sec)

select后使用子查询(了解)

显示员工名与其对应的部门名

mysql> select e.ename,(select d.dname from dept d where e.deptno=d.deptno) as dn
ame from emp e;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)